The present invention relates generally to the field of pulp and paper technology, and, in its most preferred embodiments, to the field of corrugated containerboard.
Corrugated boxes, which are used to package a variety of goods, are commonly stacked upon one another. Corrugated boxes which are stacked must have sufficient stacking strength to maintain their shape while supporting the corrugated boxes, with goods therein, stacked above them. Therefore, corrugated boxes are commonly constructed so as to utilize the strength of corrugated containerboard. Corrugated containerboard typically includes a corrugated medium that is sandwiched between a top liner and a bottom liner. Alternatively, some types of corrugated containerboard include only a corrugated medium and a top liner, while other types of corrugated containerboard include only a corrugated medium and a bottom liner. Typically, the top liner and bottom liner are planar pieces of paperboard while the corrugated medium is a fluted piece of paperboard. Corrugated boxes constructed from corrugated containerboard, with the flutes oriented vertically, have great stacking strength.
Paperboard that is acceptable for the fabrication of corrugated containerboard is fabricated by paper machines. For example, paperboard is fabricated by preparing a dilute (0.1%-1.0%) suspension of suitably prepared wood fiber in a head-box. The dilute suspension is continuously deposited from the head-box onto a moving endless belt constructed of screen, wire, or fabric. The belt is carried by at least two large rolls. Water in the dilute suspension freely drains through the belt leaving a somewhat non-uniform mat of interwoven fibers upon the belt. Water is further removed from the mat of fibers on the belt by various vacuum devices so that the mat of fibers becomes a continuous wet web that is approximately 20% fiber. As the wet web reaches the end of the belt it is sufficiently coherent to support itself across a short gap and be picked up by another endless belt constructed of felt-like material. The felt-like belt carries the web into a mechanical press or presses which remove additional water. The web is approximately 40% fiber when leaving the mechanical presses and the felt-like belt. When the web leaves the felt-like belt it is strong enough to support itself and be drawn through dryers. The dryers are heated cylinders which dry the web to form paperboard which is approximately 94% fiber.
In accordance with the above method, a continuous sheet of paperboard is formed in the "machine direction". The machine direction is defined by the direction in which fibers pass through the paper machine. In other words, the machine direction is the direction from the head-box toward and through the heated cylinders. As fibers pass though the paper machine in the machine direction, the fibers bond to form a strong paperboard. Several factors play a role in the bonding of the fibers. These include the drying that takes place and also the fact that, as the web passes through the dryers, the web is tensioned. The tension, in combination with the drying, causes the fibers to come into intimate contact, and the majority of the fibers align in the machine direction. Thus, the "machine direction" also indicates the direction in which the majority of the fibers within a piece of paperboard are aligned. As a result of the alignment of the fibers, the paperboard has a greater strength in the machine direction than it does in the cross-machine direction (i.e., 90 degrees to the machine direction).
As specified above, corrugated containerboard typically includes a corrugated medium that is sandwiched between a top liner and a bottom liner. The top and bottom liner are paperboard that is fabricated in the manner described above. The corrugated medium is constructed from a corrugating medium which is also paperboard fabricated as described above. It is typical for the fabrication of the liner and the corrugating medium to vary slightly so that the liner is stronger than the corrugating medium, and the corrugating medium is stiffer than the liner. For example, the liner is typically fabricated from pine trees whereas the corrugating medium is typically fabricated from hardwood trees. Other differences between the corrugating medium and liner should be understood by those reasonably skilled in the art.
The corrugated medium is commonly formed by passing, in the machine direction, a continuous sheet of corrugating medium through a pair of corrugating rolls which are oriented perpendicularly to the machine direction. The corrugating rolls are typically cylindrical rolls having protruding, rounded teeth. Each tooth is an elongated protrusion that runs from one end to the other of a corrugating roll such that each tooth is parallel to the axis of the corrugating roll. The teeth of the pair of corrugating rolls mesh snugly and the corrugating medium passes through a nip defined between the pair of corrugating rolls such that the corrugating medium is fluted to form the corrugated medium. Flutes add strength in the direction parallel to the flutes. Conventionally, the flutes are formed perpendicularly to the machine direction of the corrugated medium. As indicated above, paperboard (and thus the corrugated medium) has a greater strength in the machine direction than it does in the cross-machine direction. Thus, the strength that is added by the flutes is not aligned with the greater strength that is attributable to the machine direction of the corrugated medium.
Once the corrugated medium is formed, the corrugated containerboard is formed by sandwiching, and securing with glue, the corrugated medium between a top liner and bottom liner. The corrugated containerboard is conventionally formed such that the machine directions of the liners and corrugated medium are parallel. However, the flutes end up being oriented in the cross-machine direction. As indicated above, paperboard (and thus the liners and corrugated medium) has a greater strength in the machine direction than it does in cross-machine direction. Thus, the component of strength that is attributable to the flutes is oriented perpendicularly to the greater components of strength that are attributable to the machine direction of the liners and corrugated medium. Thus, the maximum potential stacking strength of the corrugated containerboard is not realized.
A conventional method of obtaining stronger corrugated containerboard is to increase the fiber content of the corrugated containerboard. The fiber content of the corrugated containerboard is increased by increasing the amount of pulp that is utilized in the process of fabricating the paperboard from which the corrugated containerboard is fabricated. Thus, the consumption of more natural resources is required which increases the cost of the corrugated containerboard and is, arguably, detrimental to the environment.
There is, therefore, a need for an improved corrugated containerboard, and a method and apparatus for fabricating the improved corrugated containerboard, which address these and other related, and unrelated, problems.